Various sublimation type thermal transfer processes are known in the art, and, in recent years, the advance of recording methods has enabled fine letters, figures, full-color, photograph-like images or the like to be formed on a thermal transfer image-receiving sheet, for example, a polyester sheet or paper bearing a dye-receptive layer, at a high speed by means of a thermal head or the like.
In the above prior art, the heating time at the time of thermal transfer by means of a thermal head should be very short. In this case, the sublimable dye and the object, on which the dye is to be transferred, are not satisfactorily heated, making it impossible to form an image having a satisfactory density. In order to cope with such high-speed recording, dyes having excellent sublimability have been developed. In general, however, the molecular weight of such dyes is so low that the light fastness of the transferred dyes in the object is unsatisfactory, resulting in fading of the formed image with the elapse of time.
When dyes having a relatively large molecular weight are used in order to avoid the above problem, the sublimation rate of such dyes is so low that images having satisfactory density cannot be formed in high-speed recording.
Pyrazolone-azomethine dyes having excellent color density are known as dyes for thermal transfer sheets (for example, Japanese Patent Laid-Open Nos. 156791/1984, 184339/1984, 19138/1985, 205288/1988, and 147893/1991). Images formed using these pyrazolone-azomethine dyes by thermal transfer unfavorably have remarkably poor light fastness.